I've shot with all the canon EF mount 50's.. the 50f1.8mk 1 mk2, 50f1.4, 50f1.2 and then the sigma 50f1.4.

For the price, that sigma is a good piece of glass. The copy I had seemed to back focus.. and I've read some other reviews that lead to the same. A quick AF-microadjust by a friend and it was great. Eventually sold it for my current 50f1.2L But I would go as far as to say that I liked the sigma 1.4 over the canon 1.4 for sure